The Add to Project command in the Project menu works like the File|Open command with two small but important differences.
If the active project is a managed project, files opened with Project|Add to Project are opened and added to the project as open files. Files opened with File|Open are opened as outside files that are not part of the project. If you close the managed project and then reopen it, files opened with Project|Add to Project are opened again along with the project, while files opened with File|Open are not remembered by the project.
If you have already opened files with File|Open and they now appear as outside files in your managed project, you can use Project|Add Outside Files to make them part of the project.
If the active project is an unmanaged project, then there is no difference between opening files using File|Open or Project|Add to Project. Both commands add the files and open files to unmanaged projects.
The other difference is the submenu with recently closed files. This applies to both managed and unmanaged projects. The File|Open submenu lists files that were opened and then closed in any project, including untitled projects that were never saved. This list is saved by EditPad along with its general settings and history information. The Project|Add to Project submenu lists files that were opened and then closed in the current project only. This list is saved in the .epp project file.
If you haven't worked with a particular project for a while, any files that you had open in that project and then closed will have long dropped off the 100 files that the File|Open submenu remembers. But when you open that project again, the Project|Add to Project submenu will still remember the last 100 files you closed while working with that project.
The File|Open and Project|Add to Project submenus remember all files that were closed even if you didn't use that menu item to open them. For example, if you double-click on a file in Windows Explorer and then close the file, it will be listed at the top in both File|Open and Project|Add to Project. If you then switch to another project, the file will still be at the top in File|Open but won't be in the Project|Add to Project submenu for the project you switched to. Files that are opened in bulk such as by using Project|Open Folder are not remembered by either menu.
Project|Add to Project Unopened
Project|Add Outside Files